1. Field of the Invention
The invention relates to a method of separating a foreground picture signal FG from a mixed picture signal MG having a key color KC and transition colors to this key color in key regions, the components of the key color and the transition colors being vectorially subtracted from the mixed picture signal in dependence upon a control signal k which, with a continuous variation, assumes values of between 1 in the region of the key color KC and 0 at mixing boundaries.
2. Description of the Related Art
In such methods the component of the key color KC which is assumed to be present in the mixed picture signal is vectorially subtracted from the mixed picture signal. This is effected in dependence upon a control signal which continuously assumes values of between 1 (full subtraction of the key color) and 0 (no subtraction of the key color). The key color component can then also be subtracted from transition colors which have only a smaller key color component (with a reduced color intensity).
The term "color intensity" is herein understood to be a distinctive characteristic feature of such colors in the component color space, which colors have in common that all of them are situated on the same straight line, starting at achromatic locus/black. Thus, hue, chromaticity and also color saturation are common because the chrominance-to-luminance ratio always remains equal.
The term "color intensity" will hereinafter frequently be used instead of two-dimensional projection in the C.sub.b /C.sub.r plane, i.e. of the chroma, because the question of whether the luminance varies or does not vary is left open by the term "chroma variation ".
However, the vectorial subtraction of the key color involves the problem that in many picture signals there is no linear relation between the luminance signal of the camera input and the chrominance signal. A gamma pre-correction is often performed which, at the display end, is corrected by the gradation of the phosphor on the display screen of a monitor. However, errors will occur if the foreground picture signal is to be separated from the mixed picture signal after the gamma pre-correction or before a gamma correction.
The cause of the deviations from the linear behavior is not the gamma correction itself, as may easily be assumed at first, but the type of gamma correction. The problem is posed by the gamma characteristic curves of the R, G, B camera signals which deviate from an ideal exponential curve.
When coding to form component signals, signals of the three channels are combined at differently large amplitudes and thus also at different gradients. Consequently, the gradient is dependent on the amplitude. However, a purely exponential variation is an exception. Though its variation is curved, its exponent (slope) is constant throughout the amplitude.
To eliminate such problems, it is known from the thesis entitled "Television Animation Store: Digital Chroma Key and Mixer Units" by V. G. Devereux in Research Report 1984/16 of the British Broadcasting Corporation Engineering Division, to define a narrow angular range including all feasible divergences of the key color and of the transition colors. After vectorial subtraction of the components of the key color and the transition colors, all remaining colors are set at their achromatic locus. This method has the drawback that it is limited to small divergention angular ranges of the key color and of the transition colors in the C.sub.b /C.sub.r color plane. Moreover, weak color nuances of a transparent object which also comprises components of the key color or of transition colors are lost.